FIG. 1 illustrates a wind turbine 1, comprising a tower 2 on which a nacelle 3 is mounted. At least one turbine blade 5 is mounted on a hub 6 to form a rotor 4. The hub 6 is connected to the nacelle 3 through a low speed shaft (not shown) extending from the nacelle front. The wind turbine illustrated in FIG. 1 may be a small model intended for domestic or light utility usage, or may be a large model, such as those that are suitable for use in large scale electricity generation on a wind farm. In the latter case, the diameter of the rotor may be as large as 150 meters or more.
The rotor blades of wind turbines are designed to extract power from the wind by virtue of their aerodynamic shape, and subsequent wind induced rotation. For horizontal axis wind turbines, the blades rotate around a rotor hub attached to a nacelle mounted on a wind turbine tower, and the rotation of the rotor turns a drive shaft connected in turn to a generator which produces electricity. For horizontal axis wind turbines to operate efficiently and extract the maximum power from the wind, the wind turbine nacelle and the axis around which the wind turbine rotor rotates, is angled into the wind to the greatest extent possible, such that the rotational axis of the rotor is aligned with the wind direction.
US patent application US 2005196280 describes a yawing system for a wind turbine, the wind turbine comprising a tower fixed to the ground and a nacelle housing an electric power generator. The tower and the nacelle are joined by the yawing system which allows the orientation of the nacelle with respect to the tower according to the direction of the wind, and requires 360 degrees of rotation to be achieved by the yawing system. The yawing system comprises a gear ring fixed to the tower for rotating the nacelle relative to the tower. The gear ring is divided into gear-toothed circular segments of uniform size.
Modern wind turbines control the load on the rotor by pitching the blades about their longitudinal axis in and out of the incoming wind. FIG. 2 shows a known pitching arrangement between a hub 3 and a blade (not shown). The pitch bearing 7 is located between the hub and the blade. A gear 9 is formed on the pitch bearing 7 and a torque is applied to the gear to turn the pitch bearing by pinion 8. The pinion is turned by an actuator or motor, such as a hydraulic cylinder or electric motor, to provide the torque for pitching the blade and maintaining it in a given position. Known pitching arrangements such as that of FIG. 2 use a pitch gear ring formed as a single piece extending around the entirety of the bearing circumference.
Pitching of wind turbine blades can be used to achieve two aims. The primary task is to control the angle of attack within what will be called the “operational range” for controlling the power and speed of the rotor when the turbine is being used to extract energy from oncoming wind. Mechanically adjusting the rotor blade pitch angle controls the aerodynamic angle of attack, and thus the input power to the generator, as is well known in the field of wind turbines. The majority of blade pitching takes place between a narrow range of around 5°. For example, if a pitch angle is taken as that between the plane of rotation of the blade tip and the blade tip chord, a pitch of 0° being the pitch at which the chord of a turbine blade tip is parallel with the plane of rotation of the blade tip, the majority of pitching may be performed between −2° and +2°. Taking into account other factors such as power control, a pitching range of around 30° to 35° is typically enough variation for controlling the blade within the operational range sufficiently, this may correspond to pitching the blade from −5° through to 30° if pitch angle is determined as described above. However, the second task of the pitching mechanism is to provide aerodynamic braking for the rotor by pitching the rotor blades fully out of the wind to reduce the lift coefficient as far as possible (known as feathering). This requires a total pitching range of around 90°-100°. The majority of the time during operation, when extracting energy from oncoming wind, the pitching mechanism will be adjusted within the power control or operational range that extends around 30° or perhaps 35° of pitching and in particular in the very narrow range of around 5° to 10°; only a small proportion of the time will the pitching mechanism need to pitch to the feathered or stall position of around 90°.
A unique problem with pitch bearings for wind turbines is that they are not operated like traditional gear systems and must be considered differently from other gear systems such as, for example, yaw systems. The pitch gear goes through long periods of inactivity, in which it is not necessary to change the pitch of the blades. When the blades are pitched, only relatively small rotations are required to control the power and speed of the rotor when the turbine is being used to extract energy from oncoming wind, this being within what will be called the “operational range” for pitching. The result is that the pinion spends most of the time engaged with, and travelling over, a short section of the gear surface. Typically, this equates to around 30°-35° of the gear surface, defined by the angle of a segment of the gear ring, with a particularly high wear section of around 5° to 10°. This leads to accelerated wear, and when the small section of gear covering adjustments in the operational range becomes worn the entire gear must be replaced. Because the gear is formed directly on the blade bearing the entire bearing, or bearing ring, must be replaced, which is expensive and difficult.
We have appreciated that the gear rings on turbine pitch control systems are subject to fatigue during use and that an improved arrangement for mitigating against fatigue damage and allowing easy replacement of a gear ring would be advantageous. In particular, we have appreciated that a new type of gear ring would be advantageous that can be installed in a wind turbine and, after installation, can be easily repaired when required